1. Technical Field
This invention relates to auto systems and, more particularly, to voice switching circuits which connect to a communication line for providing two-way voice switched communications.
2. Description of the Prior Art
The use of analog speakerphones have been the primary hands free means of communicating during a telephone conversation for a great number of years. This convenient service has been obtained at the price of some limitations, however. These speakerphone usually require careful and expensive calibration in order to operate in an acceptable manner. They are also designed to operate in a worst-case electrical environment thereby sacrificing the improved performance that is possible in a better environment.
The operation of conventional analog speakerphones is well known and is described in an article by A. Busala, "Fundamental Considerations in the Design of a Voice-Switched Speakerphone," Bell System Technical Journal, Vol. 39, No. 2, March 1960, pp 265-294. Analog speakerphones generally use a switched-loss technique through which the energy of the voice signals in both a transmit and a receive direction are sensed and a switching decision made based upon that information. The voice signal having the highest energy level in a first direction will be given a clear talking path and the voice signal in the opposite direction will be attenuated by having loss switched into its talking path. If voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an "at rest" mode which provides the clear talking path to voice signals in a receive direction favoring speech from a distance speaker. In some modern analog speakerphones, if voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an idle mode where the loss in each direction is set to a mid-range level to allow the direction wherein voice signals first appear to quickly obtain the clear talking path.
Most high-end analog speakerphones also have a noise-guard circuit to adjust the switching levels according to the level of background noise present. Switching speed is limited by a worst-case time constant that assures that any speech energy in the room has time to dissipate. This limitation is necessary to prevent "self switching", a condition where room echoes are falsely detected as near-end speech.
A disadvantage associated with the conventional analog speakerphone is that it is unable to relate to the hybrid it faces when attached to a telephone line. Even a digital telephone within a private branch exchange (PBX), which does not employ a hybrid, faces an unpredictable hybrid on calls which are routed outside of the PBX. A worst case trans-hybrid loss must therefore be assumed. This assumption requires the insertion of more switched loss than otherwise might be necessary in order to assure that the system will remain stable. A high "break in" threshold is similarly required in order to prevent a bad hybrid from reflecting enough transmit speech to falsely switch the speakerphone into the receive state. Thus the optimal performance possible with this speakerphone in its interface with a hybrid is not achieved.